GEOPHYSICAL SURVEY

Dominion, Volume 28, Issue 104, 26 January 1935, Page 8

GEOPHYSICAL SURVEY

Activities of Department Outlined PROGRESS IN DOMINION Dominion Special Service. Hamilton, January 25. An outline of the activities of the Department of Scientific and Industrial Research in geophysical survey was given by Professor 11. G. Denham in his address at the conference of chemists to-day. During the last few years, Professor Denham said, tliere had been a considerable development in all parts of tlie world in the application of geophysical methods of surveying, and the potentialities of this branch of applied science as a valuable and economic method of prospecting was becoming widely recognised. The department entered this field at the beginning of 1933, with a magnetic survey over Cromwell Flat and since then the work had been continuously developed and extended so that at the present time there were three separate groups working in different parts of the country—one in Central Otago in connection with alluvial gold deposits, one at Waihi where the problem deals with gold-bearing reefs, and one at New Plymouth studying the geological conditions associated with possible oil accumulations. Success in Prospecting. The success of geophysical prospecting depended on the existence of differences in the physical properties of an ore body and the surrounding rock, for instance, or between different geological strata and so on. Tliere were four principal methods of geophysical surveying each of which depended on a different physical property of the ma-, terials it is wished to differentiate. The magnetic method depended on differences in the magnetic susceptibility of various materials. The electrical method, of which there were a number of variations, utilised differences in electrical conductivity. The seismic method depended on variations in the elasticity of adjacent geological formations and finally the gravimetric method, iu which a torsion balance was used, depended on differences in density. In the department’s work the first three methods had ' been used ; the seismic method most extensively. The magnetic method bad been successfully used iu two types of problem. In the first, of which Cromwell Flat and Cornish Point were typical examples, it had been used to trace concentrations of black sand or magnetite which mark the course of ancient river channels while in the second, at Reefton, it had been used to trace the course of certain dikes. Applications of Electrical Method. fl'lie electrical method had very wide applications and had been used to determine underground structure, as in the tracing of u fault obscured by the Wetherstones cements, and in exploring reef country as iu the Reefton district and at present at Waihi. The seismic method had been very extensively and successfully used in the mapping of buried topography. In many' cases alluvial gold was found along the bottom of ancient river valleys which were now obscured and the seismic method had been used to trace the shape of the basement rock below the overlying sediments, sometimes three or four hundred feet deep. Extensive surveys of this type had been carried out in the Waikaia area, in parts of the Manuherikia valley, at Mahakipawa and elsewhere. “It cannot be stressed too often,” said Professor Denham, “that geophysical methods do not locate gold (or oil) directly but give information relating to the geological formation or structure which is likely to be associated witli gold. Direct methods of prospecting, such as boring, are necessary to test whether or not payable values exist in any particular case. A preliminary geophysical survey, however, enables direct prospecting to be planned and carried out with a maximum of economy and provides information enabling any subsequent development to be carried out along efficient lines.”